The present invention relates to gas turbine engines, and more particularly, to devices and systems for mitigating combustion instability in gas turbine engines.
Gas turbine engines are well known for generating power and for propelling various types of flight vehicles, including high-speed air-breathing vehicles such as ramjet and scramjet powered flight vehicles. Generally, the combustor section of the turbine engine receives fuel and an oxidizer for combustion. The oxidizer, such as air, is passed through the compressor section of the turbine engine. The compressed air stream is mixed with the fuel, which is typically introduced into the combustor section via one or more injectors or atomizers. The air/fuel mixture is then ignited by a burner causing rapid expansion within the combustor section. The expanding air from the combustor section flows through the turbine section of the engine causing rotationally mounted turbine blades to rotate and generate energy. The air exiting the turbine section is exhausted from the engine via the exhaust section, thereby creating thrust.
A significant problem in the design of gas turbine engines, particularly low-emission, high-performance turbine engines, is combustion instability. Combustion dynamics in the form of fluctuations in pressure and heat-release rate can give rise to combustion instability, which is generally considered to be the presence of high-amplitude acoustic pressure waves or vibrations within the combustor resulting from the combustion process. In addition to reducing engine efficiency and performance, these high-amplitude vibrations can damage hardware components and lead to turbine failure. The term “screech” is used in the industry to describe high-frequency pressure oscillations, which are likely to cause structural damage to the combustor and combustion components. The term “rumble” is used in the industry to describe low-frequency pressure oscillations, which create inefficiencies in turbine performance.
The two general methods know for addressing combustion instability are passive combustion control and active combustion control. Generally, passive control refers to addressing the dynamic pressure oscillations by the design characteristics of certain components, such as the shape or capacity of the combustion chamber or the pattern of fuel injectors therein. On the other hand, active control generally refers to the monitoring of the combustor environment and actively controlling combustion parameters to reduce combustion instability.
One type of active combustion control system includes introducing modulated fuel into the combustor. Generally, sensors detect combustor dynamics and provide feedback signals to a controller, which in turn operates a control valve to modulate the fuel prior to injection into the combustor. The fuel amplitude, frequency and phase are modulated so that by cancellation of waves the pressure oscillation within the combustor is mitigated or offset entirely, thereby tending to stabilize combustion. See e.g., U.S. Pat. Nos. 6,640,549 and 7,654,092 and U.S. Pub. Nos. 2002/0162336, 2008/0134684, 2009/0204306 and 2009/0026398.
Many of the aforementioned patent documents pertain to aspects of the electronic control of the modulation or the arrangement of the modulated burner assemblies, rather than the construction of the modulating control valve. Moreover, many prior art combustion control systems are not suited for operation at high frequencies, such as 1,000 Hz or more, which are prevalent in high-performance propulsion systems used in high-speed flight vehicles.
U.S. patent application publication No. 2009/0026398 describes various high-frequency fuel modulating valve constructions. This published application discloses modulating valves with oscillating latching valve assemblies. In particular, the valves have oscillating rotors, made of ferritic magnetic flux permeably material, that rotate between electromagnetically latched positions to control flow of modulated fuel to the outlet of the valve. The valves rely on springs for rotating the rotor between latched positions, such as an arrangement of coiled springs, a torsion spring or a cantilever spring. The use of springs introduces inaccuracies in flow modulation resulting from inherent variations and imprecision in spring rate(s). The springs are also difficult to assemble and leave the valve susceptible to failure. The imprecision and failure risk leave such valve constructions undesirable for critical, high-performance applications.
An improved active combustion control device is thus needed to mitigate high-frequency combustion instability, particularly present in high-performance propulsion systems.